Ion sources are used in ion beam and neutral beam accelerators, spectrometers, for ion implantation, waste control of radioactive nuclear materials, and in plasma processing. Plasma processing encompasses the use of plasmas for surface treatment or surface modification including, but not limited to, ion implantation or coating of surfaces. Ion beams may also be useful for fusion. The positive or negative ions are generally obtained or extracted from a plasma formed from a gas. The plasma is created by exciting electrons in a vacuum chamber. The gas is ionized by electron bombardment, vacuum arc discharge, thermal filaments, or power coupled from a power source to the gas via an antenna. This invention relates to a new improved radio frequency drive antenna used to create a plasma from which ions are extracted.
An ion source for an accelerator or the like generally comprises a vacuum chamber in which a low pressure gas, such as hydrogen or deuterium, is ionized to produce an ionized gas plasma. It is often desirable to produce a plasma having a high ion density so that the accelerator may be supplied with a beam having a high density of charged or neutral particles, such as gas ions, atoms or molecules.
One common method of producing a high density plasma in an ion source is to provide thermionic cathode filaments which emit a copious supply of electrons, which then may be accelerated to produce an ionized gas plasma. This approach has the disadvantage that thermionic cathode filaments often have a very short operating life, e.g., only a few hours. Moreover, the electrically heated filaments produce considerable heat which may cause operating problems, such as outgassing from the filaments or from chamber walls.
Another method of producing a dense ionized plasma is to supply radio frequency power to the vacuum space. Generally, a small thermionic cathode filament is provided to emit electrons so that there is initial ionization of the ionizable gas, which then derives additional energy from the radio frequency power, causing additional ionization of the gas. The result is that a dense ionized gas plasma is produced. The power is supplied to the ion source by an antenna in the vacuum chamber. The RF power can be thought of as heating or increasing the energy level of the electrons so that a dense plasma is produced.
A prior art plasma ion source system is shown in FIG. 1. An ion source antenna 10 is installed in a volume plasma ion source 45 having a vacuum chamber or housing 46 within which the antenna 10 is mounted. The antenna has lead-ins 15 and 16 extending through seals 98 and 99. The terminals or contacts 17 and 18 are connected to an AC power supply circuit 50 through an impedance matching circuit 51. AC power supply circuit 50 includes an AC power supply 52. The power supply 52 may have a control circuit 54 for regulating the power supplied to the antenna. The RF frequency is typically in the range of two megahertz to 14 megahertz. However, RF frequencies lower than 500 kHz may be acceptable for ion implantation systems. The plasma ion source 45 typically also includes means within the vacuum chamber 46 for producing initial ionization. Such means may take the form of a small electron-emitting filament (not shown in FIG. 1). The power supplied by the plasma ion source antenna 10 then increases the level of ionization and produces a dense plasma within the vacuum chamber from which ions can be extracted to generate an ion beam.
A typical plasma ion source 45 also has means to generate a vacuum within the chamber. A vacuum pump 56 is connected to the vacuum chamber 46 by a vacuum line 58 which includes a regulating valve 60. The vacuum pump is operative to establish and maintain an appropriate vacuum level in the chamber 46. FIG. 1 also shows a plasma gas source 62, connected to the vacuum chamber 46 by a supply line 64 which includes a regulating valve 66. The gas source 62 may be a pressure tank containing the desired plasma gas, such as hydrogen, nitrogen, or others, to be ionized in the vacuum chamber 46, so as to produce the desired high density plasma.
As shown in FIG. 1, the plasma ion source 45 provides a copious supply of ions to a beam accelerator 70. The beam accelerator 70 may generate a beam of particles or ions which are either positively or negatively charged. The current of ions extracted from plasma ion source 45 is typically referred to as an "extracted current." The maximum extractable current is proportional to the plasma density. The extracted current generally increases as more RF energy is coupled to the plasma, which increases the density of the plasma. Some of the factors that influence the coupling of the RF antenna power to the plasma include the RF resistance of the antenna and whether or not (as described below) there are high-field short circuit paths that shunt RF power across the antenna coils. A common figure of merit in ion beam systems is the ratio of extracted ion current (mA) to the RF input power (kW).
Typically, the extracted ion current plateaus at a high RF input power because of deleterious effects that limit the actual RF power coupled to the plasma. When the antenna coil is made of bare metal, such as copper, sparking or arcing may occur in the vacuum chamber, both between the turns of the coil, and also between the coil and various electrodes which may be employed in the ion source. When the antenna coil is operated at high power levels, the RF voltage between different portions of the coil may be quite high. Moreover, electrodes may be employed in the ion source to produce accelerating voltages which are quite high, so that sparking or arcing may occur. The arcing and sparking that occurs during high RF power operation limits the magnitude of the extraction current that can be obtained.
An additional problem encountered in high-density, high energy plasma systems is sputtering of the antenna surface. When a bare metal antenna coil is employed in an ion source, problems are often encountered with sputtering of the copper or other metal from the antenna coil, due to ion bombardment of the antenna coil. The sputtered copper or other metal is deposited on other surfaces within the vacuum chamber of the ion source, and may exacerbate other problems, such as current leakage or short circuits between electrodes.
There are several approaches that attempt to solve the problems of voltage breakdown, sparking, arcing, and sputtering by covering the bare copper tube of an antenna coil with an additional layer of dielectric. All of these approaches, however, are limited in their effectiveness by the types and thickness of dielectric that can be coated over a copper tube. For example, sleeving material made of woven glass or quartz fibers can be inserted over a copper tube. However, the woven glass or quartz sleeving provides only limited protection against voltage breakdown, sparking, and arcing. Consequently, the efficiency of antenna coils covered with common sleeving materials is poor at high voltages. Additionally, this approach reduces sputtering but does not eliminate it. The sleeving does not make good thermal contact with the antenna, which results in problems with overheating. Moreover, the woven glass or quartz sheathing introduces the additional problem of causing the evolution of contaminating gases, such as oxygen and water vapor, which are driven out of the woven glass or quartz material during the operation of the ion source, due to the heat generated in the ion source during normal operation.
Another attempted solution is to coat the copper coil of an antenna with a glass-like coating (a glass frit) that is baked onto the surface of a copper tube. This approach was described by one of the present inventors in U.S. Pat. No. 4,725,449, which is incorporated by reference herein. The glass coating permits the coil to be operated at higher voltages than a bare coil or a sheathed coil without sparking or arcing between coil elements. This improves the efficiency of the antenna. Another advantage is that since glass is electrically insulating, the exterior of the antenna floats at a lower negative potential than that applied to the antenna. This reduces the sputtering of antenna material by the surrounding plasma ions. However, there are several limitations to this approach. First, there are practical limitations to the type of glass frit and the thickness of the glass layer that can be applied to the surface of a copper tube. The thickness of the glass coating is typically less than 1 mm (e.g., on the order of 0.1 mm), which limits the ability of the coating to protect the antenna from arcing and sparking at high field strengths. Moreover, at high power levels, high energy electrons appear to penetrate the glass coating and significantly degrade its electrically insulating properties. The thin glass coating is also inherently fragile. Another problem at high power levels is that there may be large thermal stresses resulting from differences in the thermal coefficient of expansion (TCE) of copper and the glass layer. Additionally, even if the TCE is matched, thermal stresses can arise because of thermal gradients (e.g., differences in transient or equilibrium temperature of the copper tube and the glass layer). The thermal gradients causing thermal stresses can be calculated. Those skilled in the art of thermodynamical engineering are familiar with mathematical techniques to calculate the transient and equilibrium temperature rise in a multilayer structure comprised of an outer layer and an inner layer connected to a heat sink. The inner surface of the copper tube is in contact with the coolant. The copper is resistively heated by the RF current. The outer glass coating is heated by the copper and to a lesser extent by the plasma. At high power levels, the glass coating may crack or flake-off from the stresses caused by differences in TCE and the thermal gradients. Practically, the glass coating is not an effective electrical insulator at pulsed RF powers above about 25-30 kW.
Still another attempted solution is the use of a porcelain coating applied to a copper tube antenna. This approach is described by one of the present inventors in U.S. Pat. No. 5,587,226, which is incorporated by reference herein. The porcelain coating permits higher power levels than the use of a glass frit coating. However, at high power levels, the thermal gradients can cause flaking or cracking of the porcelain. Additionally, while in principle a comparatively thick layer of porcelain can be applied, typically less than 1 mm thick layers are utilized. Consequently, the porcelain coating is often too thin to provide the desired dielectric strength at high RF voltages. Finally, another problem with a porcelain coating is that it is unsuitable for some types of applications. For example, in semiconductor processing, the sputtering of the porcelain surface at high power levels may create an undesirable source of contamination. Oxygen plasmas can also attack the porcelain, making the porcelain antenna unsuitable for the generation of a high density oxygen plasma.
The above described attempted solutions rely upon coating a copper tube with a dielectric layer. As described above, the performance of such antennas are limited by the types and thicknesses of dielectric material that can be coated over a copper tube. The dielectric layer may not be as thick as desired at high RF powers, leading to reduced efficiency. Thermal stresses on the dielectric coating and/or attack by the plasma may limit the operating conditions with which the antenna can be used.
A fundamentally different attempted solution to creating a high efficiency RF antenna is to insulate a metallic conductive element inside a hollow glass or quartz tube. This approach is hereinafter referred to as the "glass tube" antenna approach. As shown in the prior art cross-sectional schematic of FIG. 2, in conventional glass tube antennas the coil is comprised of a glass tube 20 containing an inner conductor 18b. The glass tube 20 enters the top of the vacuum chamber 14 through wall entrances 22. In the center of the vacuum chamber 14 the glass tube has the shape of coils 18a. In conventional glass tube antennas the conductor 18b may be formed from either a conducting layer precipitated onto the walls of the tube 20 or it may be a conductive "chord" inserted into the tube. The glass tube antenna approach has the advantage that the glass can be thick enough to protect the conductive elements of the antenna from shorting or arcing at high field strengths. Additionally, high energy electrons are less likely to degrade the dielectric properties of a comparatively thick glass tube. Coolant may also be flowed through the glass tube 20. Unfortunately, conventional glass-tube antennas have several problems that limit their efficiency and make them impractical for use in common plasma systems, particularly at high power levels. Conventional glass-tube antennas are typically not used in high-power density plasma systems even though the potentially thicker dielectric layers may reduce arcing and sparking at high RF powers.
Many factors have prevented the widescale use of glass-tube antenna structures. One problem is that in many applications, such as proton therapy, boron neutron capture therapy (BNCT), neutron radiography, and spallation neutron sources, the antenna must be positioned in the central discharge chamber where the uniform plasma density is produced. Consequently, the reliability of the antenna is a major concern. Another problem the limits the usefulness of conventional glass tube antennas is the thermal problems that hinder achieving a compact, high-power antenna design. For example, in many applications it is desirable to reduce the radius of curvature of the antenna coil and decrease the outer diameter of the tubing used to fabricate the coil. However, a small diameter glass tube will have a relatively high resistance to fluid flow, which may limit the flow of a coolant. Although the coolant pressure can be increased somewhat to increase the fluid flow rate, a small diameter glass tube with comparatively thin walls will burst if the pressure is raised too high. Consequently, the maximum flow rates of coolant must decrease as the tube size decreases. However, as the tube size is decreased, the cross-sectional area of the enclosed conductive element must also decrease. This increases the resistivity of the conductive element, which leads to a greater heat load. A combination of reduced coolant flow rates, a higher heat load, and weaker tube walls all exacerbate the problem of preventing the glass tube from cracking because of thermal stresses. Consequently, a conventional glass tube antenna may be thermally limited to RF powers substantially below what is desirable for a plasma system, particularly if the glass-tube antenna is reduced to a size comparable to those of conventional porcelain coated copper tube antennas.
The inventors' own experiences with the design of antennas for RF plasma systems leads them to believe that those skilled in the art are reluctant to use conventional glass-tube antennas because of the concern that such an antenna may catastrophically fail, exposing the vacuum system to atmospheric gases and coolant. This is true even if the antenna is not driven to its thermal limits. Glass is more fragile than copper. It is inherently more difficult to design a self-supporting glass-tube antenna structure that can support its own weight and survive mechanical vibration and shock without fractures developing in the antenna or at its feedthroughs.
What is desired is a compact, efficient, high-power glass tube antenna and an antenna assembly design that facilitates operating the antenna reliably at high RF powers with a low probability of catastrophic failure.